The present invention relates to laminated metal seal rings for valves handling high pressure, high temperature and/or highly corrosive fluids and, in particular, to such valves with a laminated metal seal rings.
Expired U.S. Pat. No. 3,442,488 discloses a disc valve in which a disc is pivotable between an open position, in which the disc is substantially parallel to an axis through the housing, and a closed position in which the disc is generally transverse to the housing. The pivot axis of this disc is offset with respect to the main axis through the housing. This patent discloses to install the seal ring on the disc.
U.S. Pat. No. 4,281,817 discloses a similar valve in which the seal ring is in the housing and includes a radially inwardly projecting sealing surface which engages and cooperates with a peripheral sealing surface of the disc to form a fluid-tight seal when the disc is in its closed position. As is typical for such valves, the housing includes an enlarged diameter recess in a portion of the flow passage through it which extends from an end of the housing to an axially oriented seal ring support surface. The seal ring is placed against this support surface so that its sealing surface faces radially inward and a retainer ring, also disposed in the enlarged diameter recess, is secured to the housing.
In an improved version of the valve disclosed in the '817 patent available from the assignee of the present application, the seal ring mounting retainer ring is mounted to the housing without bolts and is spaced from the seal ring. A plurality of set screws carried by the retainer ring are tightened against the seal ring, directly or via an intervening pressure ring, to force the seal ring against the support surface of the housing and to maintain it in position.
Seal ring installations as described in the preceding paragraph work well so long as the seal ring is constructed of a compressible material, such as elastomers, and is subjected to relatively low pressure, low heat and/or is used with non-corrosive fluids. For high temperature or high pressure applications, or when used with corrosive fluids, the seal ring must be constructed of metal, such as stainless steel laminations. It is difficult to prevent leakage, or at least the seepage of some pressurized fluid past the abutting lamination surfaces, particularly when relatively higher fluid pressures are encountered. In the past this was readily solved by placing thin layers of an asbestos material between the opposing lamination surfaces. Once the axial mounting force is applied to the laminated seal ring, the asbestos layer between the laminations prevents fluid from seeping past it.
Because of the well-known health hazard presented by asbestos, it is no longer acceptable as a material for placement between the laminations to prevent leakage past them.
The simple application of a greater axial force against the seal ring laminations to prevent any fluid seepage between them is not feasible because even microscopic surface irregularities on the laminations can lead to leakage. Moreover, the laminations are thin; they normally have a thickness of no more than 1 mm and, typically, they have thicknesses less than that. Such thin laminations have no rigidity in the direction in which the holding forces are applied. This can lead to a slight warpage or deflection of the laminations over their circumference which can again lead to fluid seepage between the laminations.
As a result thereof, it continues to be necessary to place a gasket material between the laminations of the seal ring. The only feasible material capable of withstanding the high temperatures and resisting corrosion from many corrosive fluids handled by such valves is graphite. However, graphite is an essentially amorphous material which, when subjected to high pressure, acts like powdered, compressed coal without structural integrity and, therefore, has a tendency to "float" between the laminations. Thus, it is difficult to maintain a leakproof seal between laminated metal seal rings. Even when a seal was initially established, repeated cycling of the valve between its closed and open positions degenerated the graphite gasket between the laminations, which, after relatively short service times, led to leaks.